Close-coupled electrical transformer



Dec. 26, 1950 w. H. THURSTON 2,535,554

CLOSE-COUPLED ELECTRICAL TRANSFORMER Filed Jan. 24, 1949 3 Shasta-Sheet1 Dec. 26, 1950 RSTQ 2,535,554

CLOSE-COUPLED ELECTRICAL TRANSFORMER 3 Sheets Sheet 2 Dec. 26, 1950 w, us o 2,535,554

CLOSE-COUPLED ELECTRICAL TRANSFORMER Filed Jan. 24, 1949 5 Sheets-Sheet3 Patented Dec. 26, 1950 CLOSE-COUPLED ELECTRICAL TRAN SFQRMER William:1. Thurston, Contra Costa, calm, as-

signor to Shell Development Company, San Francisco, Calii'., acorporation of Delaware Application January 24, 1949, Serial No. 72,380

4 Claims. (Cl. 175456) This invention relates to step-up or step-downelectrical transformers having extremely high coupling between theprimary and secondary instruments, such as in apparatus for determiningmagnetic susceptibilities of substances by subjecting them to magneticfields of high intensities, e. g., in excess of 100,000 gausses; invideo circuits wherein a very flat frequency response is desired; invarious industrial appliances requiring current impulses, such aswelding transformers and magnet chargers, in applications requiringvoltage impulses, and for impedance matching in impulse circuits andwide frequencyrange circuits.

When the primary and secondary windings of a transformer are not closelycoupled undesirable leakage flux and leakage reactances are experienced.It is recognized that in ordinary transformers having separatedwindings, parts of which may be appreciably remote from the core, someof the magnetic flux produced by the magnetomotive force in the primarycoil will follow paths that lie wholly or partly outside of the magneticcore and that do not link with the secondary coil, and some of the fluxproduced by the back-magnetomotive force in the secondary coil will notlink with the primary coil. It may be shown that these leakage fluxes,as they are called, are equivalent to inductances in series with thewindings. Furthermore, the leakage fluxes may follow paths that liewholly or partly within the very conductors that give rise to them,thereby generating eddy currents in the conductors, especially where thelatter are of large cross-section and carry heavy currents. The eddycurrents represent a parasitic load, and also, since they flow in oneturn paths of finite resistance, an effective resistance is reflectedinto the windings. Also. the eddy currents generate heat.

The efl'ects of losses and increased impedance due to the leakageinductances and eddy currents are particularly important if atransformer is to transmit high intensity impulses of short. duraon orhigh frequencies in the audio and ultraaudio regions. Therefore, in thedesign of many impulse transformers, e. g., a 5 to l. step-downtransformer, it is important that (a) the coupling, or the intimacybetween the primary and secondary windings, should be exceptionallyclose in order to reduce the spaces available for flux leakage paths,(b) that the conductors should be thin, or else laminated, to stifleeddy currents by reducing the dimensions or continuity of the conductingmetal in directions parallel to the residual leakage flux paths, and (c)that each conductor of each winding closely surround the core with nomore gap than is necessary for insulation alone.

It is, therefore, an object to provide an improved transformer havingimproved electrical and magnetic characteristics for transmitting highintensity current or voltage impulses of short duration.

Another object is to provide a transformer that will transmit anexceptionally wide range of audio and ultra-audio frequencies with amini mum of non-linearity.

Another object is to provide a transformer hav ing an exceptionallyclose coupling between the primary and secondary windings to eliminateleakage flux as far as possible.

Another object is to provide a transformer of very high coupiing andmade of heavy windings for transmitting high intensity impulses whereinthe windings are constructed to minimize residual leakage flux paths.

Another object is to provide an improved mechanical construction forinterleaved electrical transformers in. which all conductors are made ofsimilar or identical smooth fiat stamp ings. die-castings, sheets, orleaves without folds. edgewise bending or butt .ioints, and in the formof interrupted annuli, interconnected by bridges, which can befabricated and assembled incxpeno sively, and which can be readilyclamped together.

Another object is to provide a transformer in which leakage flux isminimized by having each conductor of each winding conform very closelyto the core with only such gap as is required for insulation.

Another object is to simplify the insulation oi transformer turns byproviding a construction principle that allows all insulation to be inthe form of sheet stampings or dip or spray coatings.

Additional objects are: To provide a transformer in which the currentdensities in both primary and secondary turns are automatically thesame; and to provide a transformer having a minimum distributedcapacitance in each winding because the turns of one winding act asshields between the turns of the other winding.

Other objects will become apparent from the following description. takenin connection with the drawingforming a part of this specification andillustrating. by way of example, a preferred embodiment of theinvention. In the drawing:

Fig. 1 is a side elevation view of a. transformer according to theinvention;

Fig. 2 is a fragmentary perspective view showing the relativearrangement of some of the winding leaves and the insulating sheets(both shown thicker than in actual construction), the core beingomitted;

' Fig. 3 is a plan view of an insulating sheet;

Fig. 4 is a perspective view of a bridge;

5 is a fragmentary enlarged sectional view take ,on line 5-5 of Fig. 1,showing the arrangement of the leaves, insulating layers and thebridges; and

Fig. 6 is a schematic wiring diagram indicating the circuit arrangement.

' Briefly, according to the present invention there is provided a coreof magnetic material having a winding leg about which there are stackeda plurality of thin, flat, preferably smooth winding leaves ofconductive metal in the form of open or interrupted annuli, separatedfrom each other and from the core by intervening layers of insulatingmaterial. The winding leaves are parallel to each other, i. e., theirsurfaces are perpendicular to the axis of the winding leg of the core,and each winding leaf forms a single turn of one of the windings. Leavesforming turns of the secondary winding are interleaved between thoseforming turns of the primary winding and evenly distributed along thelength of the primary winding, regardless of the relative numbers ofleaves in the two windings. Adjacent turns or winding leaves of eachwinding are interconnected by bridges to form separate primary andsecondary windings. In the preferred embodiment, which is illustrated,the two windings contain the same number of leaves regardless of theturns ratio of the transformer, and the desired turns ratio is attainedby omitting bridges on one (or both) of the windings at equally spacedpoints along the length to subdivide such winding (or windings) into aplurality of fractional windings having winding leaves whose number isan aliquot fraction of the total number of leaves in the compositewinding; these fractional windings are' then connected in shunt bysuitable electrical conductors.

The leaves may be clamped together by tie rods or frameworkshavingengaging means with thi at H on' it are a plurality of winding leaves itmade of thin sheet material of conductive metal, preferably copper,separated bylayers M of insulating material. These layers are quite thinand their arrangement is shown more clearly in Fig. 2. While anyinsulating material may be used, such as material which is coated to oneor both sides of the leaves, it is preferred to use thin sheet materialof high dielectric strength. it was found that empire cloth is suitablefor this purpose when the leaves are carefully finished to remove allburrs and are perfectly flat; however, laminated sheet material,reinforced for greater mechanical durability, such as the productavailable commercially under the trade name Armatite, consistingessentially ofa layerof empire cloth bonded to heavy fish paper, is moregeneral- 137 suitable.

The winding leaves for the primary and secondary windings may be ofidentical construction; as seen in Fig. 2, they are thin sheet copperstampings forming interrupted annuli, i. e., providing an insulating gapill, the central opening being of sufilcient size to accommodate thecore I I. A pair of contact lugs 15 and 16 having holes ll extend to oneside on opposite sides of the a parallel lugs on the same side of eachleaf. The

winding; the tie rods are, in this case, insulated to avoid electricalcontact with the lugs and bridges.

insulating sheets may be continuous or may have slits 20 to facilitateassemblyabout the winding leg of the core. The tabs l8 and I9 have holes2| located for juxtaposition with the holes ll. For the sake of clarityboth the winding leaves and insulating sheets are shown in Fig. 2greatly thickened in relation to length and breadth; typical dimensionsappear in the last paragraph hereof. The winding leaves are stackedabout the winding leg in alternate orientations, whereby alternateleaves, which form the primary winding and are indicated by the letter12 in Fig. 2, extend in one direction, and the alternate leaves, whichform the secondary winding, and are indicated by the letter s, extend inthe opposite direction. In Fig. 1, the lugs of the leaves forming theprimary winding are indicated at 22 and those of leaves forming thesecondary winding are indicated at 23. The insulating sheets 13 are alsostacked in alternate orientations, causing the tab 18 of one sheet tolie over the contact lug of each primary leaf and the tab iii of thenext sheet to be over the contact lug E6 of the primary leaf. Sheets l3whose tabs i8 are over lugs E5 of the primary winding have their tab l9fitting over lugs i6 of the secondary leaves, while the sheets havingtabs l8 covering lugs 16 on the primary leaves have tabs I9 coveringlugs 15 on the secondary leaves.

Adjacent leaves of the same winding are interconnected by bridges 24formed of stampings of conductive sheet material such as copper. Theyare bent to provide parallel contact portions 25 and 26 which are offsetfrom each other, i. e., lie in different planes, by a distance to permitthem to lie in fiat engagement with the con- Referring to the drawingsin detail, a magnetic core, which may be of any suitable magneticmaterial, construction and shape, is shown at Ill. The transformer coreshown may be formed of numerous laminations and to form a closedmagnetic path, although a core having gap may be used. The winding legof the core is indicated tact lugs and positioned by the tabs [8 and I!as shown in Fig. 5. As shown, one contact portion 25 is in flatengagement with the bottom of a (first) contact lug IS on a windingleaf, and the other contact portion 26 is in flat engagement with theupper face of other (second) contact lug l6 of the adjacent leaf of thesame winding. Because of the alternate orientation of the insulatingsheets 13 there is a tab l8 or 19 on the face of each contact lug l5 orl6 that is not engaged by a bridge. The contact portions and 26 areprovided with holes 21 which may be aligned with the holes ll of thecontact lugs. End plates or caps 28 and 29 of rigid material are inelectrically insulated engagement with the outmost leaves and areclamped together by means of insulated tie rods 30 whichextend throughthe holes l1, 2| and 21, thereby conveniently retaining the leaves,insulating sheets and bridges in alignment.

To insure contact between the bridges and contact lugs either or bothshould be thickened slightly, or the tabs l8 and 19 should be twice thethickness of the body of the insulator. In the embodiment shown thebridges have their contact portions 25 and 26 of thickness equal to thesum of the thickness of one leaf and one insulating sheet.

The contacting faces of the lugs I5 and I6 and of the contact portions25 and 2B of the bridges may optionally be tinned before assembly andsweated together after assembly to insure good electrical contact.

In the embodiment illustrated the transformer has a 3:1 turns ratio.Hence, all adjacent pairs of leaves of the primary winding areinterconnected by bridges. Two bridges are om'tted from the secondarywinding at the third points, indicated at 3| and 32, thereby subdividingthe secondary winding into three fractional windings, each having fourturns, i. e., one-third of the number of leaves of the primary winding.These three fractional windings are connected in shunt by the circuitindicated in Fig. 6, thereby attaining an overall turns ratio of 3:1. Bythis arrangement the turns of the primary winding are connected inseries, and the turns of the secondary winding are connected in aseries-parallel combination. Other arrangements, involvingseries-parallel connections in both windings are, of course, alsopossible. It will be understood that actual transformers may have manymore than twelve leaves for each winding. 1

The conductors for effecting the series-parallel arrangement areindicated by heavy flat bars 33 and 34 in Fig. 1. These are connected tothe contact lugs of the leaves where the bridges are omitted and haveholes so as to be secured by the tie rods 2?. Thus, bars 33 engage theupper faces of contact lugs 16, and bars 34 engage the lower faces ofcontact lugs IS. The free contact lugs at the ends of both windings are,similarly, connected by heavy fiat bars 35, 36, 31 and 38, havingsimilar holes for receiving the tie rods, and adapted for connectionwith external circuits. not shown. The connectors 33, 35 and 37 areconnected to the tabs N5 of their respective leaves, and the connectors34, 36 and 38 are connected to the tabs l5 of their respective leaves.

It will be seen that the construction described affords excellentcoupling in that the primary and secondary leaves can lie fiat againstone another, spaced apart only by the thickness of the insulating layer,such as the sheets l3 or other painted or sprayed insulating material.All local double thickness of the leaves, such as would occur whenstrips of metal are folded to build up the windings and would result inincreased axial spacing, is avoided thereby. This results in a greatlyimproved space factor and better coupling.

A further advantage of the use of entirely fiat Ill] leaves or stampingsis that the current does not go around folds or sharp bends, which wouldprovide inductive paths.

Further, it is feasible with this construction to form the leaves toconform closely to cores of standard or readily manufacturedcross-sectional shapes, whereby the air gap between the core and windingcan be made extremely small. Also, the leaves can be readily constructedto surround the core closely on all sides, leaving only a minor spaceopposite the insulating gap l4. Finally, by avoiding the use or need forwrapped or discontinuous insulation labor costs during manufacture arereduced and a thinner layer of insulation can be used because theinsulation sheets 13 or other equivalent fiat, continuous insulatinglayers can be pretested and, hence, do not require the use ofspace-consuming excess insulating material. The use of such thininsulating material is facilitated because the leaves are flat andpresent no local areas at which insulating material would be subjectedto higher than average mechanical stress or wear.

The transformer described above results in extremely close coupling andlow residual leakage flux, and is suitable for producing high intensityimpulses. Thus, by using a transformer with a 5:1 step-down turns ratio,having copper leaves 0.056 inch thick, 1 inch wide, with insulation0.013 in thickness, 30 leaves in each winding, and a core cross-section1 /4 by 3 inches. magnetic field intensities of approximately onemillion gausses have been obtained when a microfarad condenser chargedto 1500 volts was discharged through the primary and the secondary wasconnected to an air core work coil having twenty turns of by /2 inchcopper edgewise wound into a coil measuring inch I. D., 1% inches O. D.,and 3 inches long.

I claim as my invention:

1. An interleaved, closely coupled transformer including a magnetic corehaving a winding leg,-

, outwardly from the winding at a different peripheral location fromthat of the lugs on the secondary winding all the lugs of the primarywinding being aligned in register with each other and all the lugs ofthe secondary winding being aligned in register with each other, and aplu rality of bridges of conductive sheet metal for each winding havingengaging portions shaped for flat engagement with alternate lugs of adjacent leaves of the same winding for connecting said leaves in series.

2. An interleaved, closely coupled transformer including a magnetic corehaving a winding leg, a primary winding and a secondary winding, each ofsaid windings having: a plurality of substantially flat, thin windingleaves of conductive metal closely stacked parallel to one another andsurrounding said winding leg, each winding leaf forming a single turn ofone of said windings and being shaped as an annulus fitting closelyabout the winding leg and having an insulating gap,

aesasss the leaves of the primary winding being alternated with an equalin number to the leaves of the secondary winding, each leaf havin afirst and second contact lug extendin to one side thereof on oppositesides of said insulating gap, the contact lugs on the leaves forming theprimary winding having a difierent peripheral location on the side ofthe winding from the location of contact lugs on leaves forming thesecondary winding, and all corresponding lugs on adjacent leaves of thesame respective windings being aligned in register with each other; aplurality of bridge for each winding interconnect= ing pairs of adjacentwinding leaves of the same winding in series and made of conductivesheet metal bent for flat engagement with a first contact lug of oneleaf and for flat en agement with the second contact lug of the adjacentwinding leaf of the same winding, one or more bridges being omitted fromat least one of said windings at evenly spaced intermediate pointsthereof to subdivide said winding into a plurality of fractionalwindings whose turns are an aliquot fraction of the total number ofleaves therein; and means for interconnecting said fractional windingsof the same composite winding in parallel.

3. In an interleaved, closely coupled transformer having a plurality ofwindin leaves shaped as flat broken annuli forming single turns of theprimary and secondary windings stacked.

about a magnetic core and separated by sheets of insulating material,the improved construction wherein all winding leaves are constructed ofstampings of thin conductive sheet metal of the same outlines, each leafhaving an insulating gap and a pair of contact lugs extending outwardlyon one side and a central cut-out portion for receiving the core, theleaves of the primary having like orientation and the leaves of thesecondary winding being interleaved with the primary winding leaves andhaving like orienta tion which is diiierent from that of the primarywindin leaves; all of. the insulating sheets are of the same outlinescorresponding to the winding leaves but having two tabs, one matchingthe location of one of the contact lugs of the primary winding leavesand the other matching one of the contact lugs of the secondary windingleaves in assembled positions, alternate insulating sheets havingalternate orientations to cause the tabs to fit over difierent pairs ofcontact lugs; and bridges are provided in each winding be tween contactlugs of adjacent leaves of the same winding, said bridges being disposedto connect the leaves of each winding in series.

4. The transformer accordin to claim 3 wherein said contact lugs,bridges and tabs are provided with holes in alignment, and the saidparts are maintained in position and clamped together by means ofelectrically insulated tie rods passing through said holes.

WILLIAM H. THURSTON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 316,354 Gaulard et al Apr. 21,1885 2,474,395 Early et a1. June 28, 1949 FOREIGN PATENTS Number CountryDate 171,836 Great Britain Dec. 1, 1921 887,228 France Nov. 8, 1943

